Are you getting enough? Sunshine is a rare commodity in the UK, and that could be bad for our health. Photograph: Jill Mead

A study that emerged from Denmark this week may not have seemed particularly striking in itself. It does, however, add to a growing body of evidence that the paucity of sunlight in the UK for most of the year not only makes us miserable, but could actually be doing us harm.

The study showed vitamin D tablets could lower blood pressure just as well as prescription medicines. It was small, but the findings were significant. Almost everyone with high blood pressure in the study had vitamin D levels that were too low. So the tablets were replacing something that was missing.

Recognition of the importance of vitamin D for our health is growing. Many of us are, however, vitamin D deficient, because the main source of it is UV radiation from the midday summer sun, absorbed through our skin – and even when it shines good and strong, these days most of us cover up or hide ourselves away.

When early man and woman made their slow but inexorable progress, over thousands of years, out of the heat of the African jungle and into the northern hemisphere, they could never have suspected that their flight from the sun could store up problems for the health of future generations. But it may be so.

Low vitamin D levels are responsible for rickets, a disease causing bowed legs in children that most of us associate with urchins in Dickensian streets and presume has been long banished from modern Britain. And now there is growing evidence that a shortage of vitamin D may make us more susceptible to other health problems, from heart disease and cancer to multiple sclerosis.

Far from disappearing, rickets is on the rise, as the recent tragic case of baby Jayden Wray made clear. His parents, Rohan Wray and Chana Al-Alas, were accused and acquitted of child abuse after what some experts said were deliberately inflicted fractures of the four-month-old baby's bones were found by others to have been caused by rickets. The baby may well have been born with the disease – and even suffered damage to his bones as he travelled down the birth canal.

Al-Alas, his mother, was vitamin D-deficient. So is an extraordinary 25% of the UK population, according to nutritionist Dr Carrie Ruxton. She classifies deficiency as "the level below which we can't sustain normal bone health" – and the reason for this mass deficiency is that from around October to March or April, even when we see the sun over the UK, we are not getting the right sort of UV rays. They are not of the wavelength necessary to make vitamin D in contact with our skin (which will anyway be mostly covered against the cold).

And even in the summer, our modern, sedentary, computer-fixated, office-based lifestyle compromises our chances. "Six to eight months of the year, we are OK, but you have to get out between about 10am and 2pm. How many of us do that if we are working in an office? Cloud reduces the sunlight and so does smog, pollution, certain clothes, moisturising creams containing UV protection and lathering children in suncream," says Ruxton.

She lives in Fife, Scotland, which gets less of the right sort of sunshine than England. It is by now well-recognised among medical authorities that vitamin D deficiencies exist. In January this year, the four chief medical officers (for England, Scotland, Wales and Northern Ireland) sent out a reminder to all GPs and other health professionals that vulnerable groups need vitamin D supplements. That includes all pregnant women, those who are breastfeeding, toddlers and the elderly who are prone to fragile bones. The reminder followed the death of a three month-old baby from a septic inflammation of the heart in which vitamin D deficiency played a part, according to a pathologist at the inquest. The north London coroner Andrew Walker wrote to the health secretary last December, asking that all pregnant women should be advised of the need to take supplements.

But, says Ruxton, the guidance has been around for years. In France, pregnant women are routinely put on vitamin D. That doesn't happen here. "I felt quite cross because I had a child about 11 years ago and nobody told me about taking vitamin D, even though it was the policy then," says Ruxton.

"Even when I was pregnant with my daughter in 2008/9, nobody told me about it. It is absolutely shocking that the information is out there and yet most pregnant women are not being told about it at all."

Most of us may assume our bodies function perfectly well if we eat a good diet and don't drink too much, smoke or take drugs. It comes as something of a shock to be told most of us have a deficiency that we can't easily compensate for without moving to Africa or taking supplements. Diet alone cannot supply the vitamin D we need. Well, it could, but we would have to eat uncomfortably large quantities of eggs, oily fish and liver. Even then, we probably would not get the optimal vitamin D levels.

The recommended dose of vitamin D in the UK for vulnerable groups is 10 micrograms a day. "Even people who eat those foods regularly only get three, four or five micrograms a day, says Dr Gail Goldberg, senior research scientist with the Medical Research Council's Nutrition and Bone Health Research Group. "Not many foods contain vitamin D naturally." Eating herrings and other oily fish in Scotland will have helped compensate for the absence of UVB in the past, but diets have changed.

The problem is even greater among people with darker skins and those who, for cultural or religious reasons, cover themselves all year round. "It used to be thought that very dark-skinned people couldn't make vitamin D, or made less," says Goldberg. "They have the same capacity. They just need longer exposure to the sunlight to make vitamin D." Scarce strong sun in the UK is, therefore, even more of a problem for them.

But all of us in the UK are struggling to make enough, and some criticise dermatologists for their campaigns to make us cover our skin, seek the shade and coat ourselves in sunblock. Ruxton puts her toddler out in the sun with no cream for 10 to 15 minutes every day. Public health guidance does now suggest we should all have our daily 10 minutes, cream-less, in the sun, but the fear generated by skin cancer warnings lingers on.

But if vitamin D has become a hot topic, it is because of a growing belief that a great deal more is now at stake than the health of our bones – important as that is, especially in small children and the elderly. The blood pressure study was one signal. Other work has suggested vitamin D could be a powerful weapon in helping the body fight infections such as tuberculosis. It is recognised to play a role in the immune system. Some studies have suggested it could help prevent some cancers, but there is no clear evidence as yet. "There is a huge amount of work going on in this whole area of vitamin D and health," says Goldberg.

Perhaps the most dramatic hypothesis is that low vitamin D levels may be a factor in multiple sclerosis, the devastating degenerative neurological condition. One of the most powerful arguments is geography – there are significantly more cases at northern latitudes, such as Scotland.

Professor George Ebers of the Nuffield Department of Clinical Neurosciences at Oxford University is one of a group of international scientists who believe Scotland's food should be fortified with vitamin D to try to cut the MS rate. His work on human genetics, showing connections between the inability to make vitamin D and MS, supports the observational evidence.

Ebers quotes the famous epidemiologist Austin Bradford Hill, who worked with Richard Doll on establishing that smoking was the cause of most lung cancers. "To explain the pronounced excess of lung cancer in any other environmental terms requires some feature of life so intimately linked with cigarette smoking that such a feature should be easily detectable," said Bradford Hill in January 1965. That is where we are at with MS and vitamin D, says Ebers. If it is not vitamin D deficiency, then it must be something linked with it. Ebers says something needs to be done beyond advising vulnerable groups to take supplements. "The CMO's office is telling people to do it, but most people aren't doing it," he says.

But the current establishment view is that caution is the best option. Scotland's chief medical officer, Harry Burns, wants broader scientific concensus: "Dietary supplements can have harmful as well as positive consequences, and recommendations need to be made on the basis of evidential benefit in well-conducted, randomised studies in large populations.

In fact, there is some fortification of food in the UK already. Margarines, spreads and some cereals contain added vitamin D. That does not have the impact of the fortification of milk, which happens in the US, say critics.

Then there is the controversial issue of how much vitamin D we need. The supplement of 10 micrograms a day recommended in the UK for pregnant women and the elderly is based on bone health. Ebers and others say we need more. "We know that in places where MS is rare, like Queensland, vitamin D levels are two to three times higher than here, and the rate of MS is one-sixth of what it is here. If we could convert the UK to the vitamin D status of Queenslanders, we would reduce MS by 80%, presumably," he said.

Higher doses of vitamin D are safe, insists Ebers, who says that at a meeting of neurologists and other scientists working on vitamin D in Chicago last year, they were all taking 4,000 international units (the north American measure, which equates to 100 micrograms) a day – and so were their families.

Vitamin D toxicity, he says, is a myth. "You can get a rise in urinary calcium, but you have to get to 40,000 international units a day even to get that. Everybody had been saying for 50 years that it is toxic, and it isn't. The worry of toxicity is hugely overblown. If you can stand out in the sun and make several thousand units of vitamin D in a matter of minutes [in Africa], how would nature have designed us in such a way that we would get sick in a few hours?"

The white skins of northerners are "almost certainly" driven by the need to absorb more sunlight as our ancestors left Africa, he says. Women with the darkest skins made the least vitamin D, so had softer bones, meaning the pelvis broke in labour, killing mother and child. "There was no more effective form of natural selection that is known," says Ebers.

Vitamin D is now on the scientific and political agenda. The government's scientific advisory committee on nutrition (SACN) is conducting a major review of the evidence to decide whether recommended levels should be higher, and whether other health issues beyond those of our bones should be taken into account.

But SACN will not report until 2014. In the meantime, as we emerge from the winter months, having used up the approximately three months' worth of stored vitamin D in our bodies from last summer, large numbers of us are arguably vitamin D deficient. Some are taking matters into their own hands. Health food retailer Holland & Barrett says sales of vitamin D at its 700 high-street stores have grown by more than 50% year on year, with some products selling four times the level they did in the same period in 2011. Otherwise, it's back to boiled eggs with a helping of liver.

• This article was amended on 7 May 2012 to correct a conversion of international units into micrograms.